Resin foam sheets have conventionally been used in a wide range of applications such as dust-proof and waterproof materials. When using these dust-proof and waterproof materials as gaskets in particular, in addition to being required to demonstrate high levels of dust resistance and water resistance in terms of basic performance, these materials are also required to demonstrate cushioning and light weight (reduction in materials used).
Under such requirements, there are dust-proof and waterproof materials such as microcellular urethane-based foam that is lowly foamed and has an closed-cell structure, foam obtained by compression-molding of highly foamed urethane, and polyolefin-based (for example, polyethylene-based) foam having closed-cells.
Among these, as a foam sheet that currently demonstrates the most favorable level of performance, a foam sheet has been proposed that has an average cell diameter in the machine direction (MD) of 110 μm to 305 μm and an average cell diameter in the transverse direction (TD) of 100 μm to 370 μm and is composed of a foam produced by going through steps including: crosslinking a foamable polyolefin sheet, foaming it thereafter, and stretching the resulting foam sheet in the MD and/or TD direction to stretch the cells in the foam sheet while maintaining a semi-molten state exhibited during foaming (Japanese Patent No. 4578407, hereafter to be referred to as Patent Literature 1).
Furthermore, MD (machine direction) refers to the direction parallel to the longitudinal direction of a foam formed into a long sheet, while TD (transverse direction) refers to the direction perpendicular to longitudinal direction of a foam formed into a long sheet.